[tex]\displaystyle\bf\\2x^{2} -7x+b=0\\\\x_{1}=3\\\\Teorema \ Vieta:\\\\x_{1} +x_{2} =\frac{7}{2} \\\\x_{1} +x_{2} =3,5\\\\x_{2} =3,5-x_{1} =3,5-3=0,5\\\\\boxed{x_{2} =0,5}\\\\\\b=x_{1} \cdot x_{2} =3\cdot 0,5=1,5\\\\\boxed{b=1,5}[/tex]